ИСТИНА

The growth of the efficiency of polymer solar cells (PSC) in recent years can be accounted to continuing development of numerous new donor and acceptor components for the active layer. Optimization of the morphology of this layer is a very important but time-consuming task. One of the most studied optimization approach is to anneal the freshly prepared blended films thermally. In-situ monitoring the blend morphology could help to find the best annealing protocol. The Raman spectroscopy can be utilized for this purpose. It was shown in several works that Raman spectrum is very sensitive to the molecular level morphology of conjugated polymer such as P3HT. Based on recent achievements in this field we propose in-situ technique for probing the degree of molecular order in organic solar cells. Raman spectra are measured during film annealing inside a temperature controlled stage and then fitted by a superposition of two components - amorphous and quasi crystalline. From these data the fraction of the quasi crystalline polymer phase is evaluated giving a dynamics of polymer ordering upon annealing. Various fullerene derivatives (methanofullerenes, metal complexes of fullerenes, fluorine fullerene derivatives) were studied as an acceptor component for PSC. We show that several fullerene types act similarly to most-studied PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), which decreases the polymer order by intercalating in its phase. Annealing is turned out to be efficient to achieve the partially crystalline polymer domains that are beneficial for solar cell performance. The developed technique is used to determine the optimum annealing parameters by in situ monitoring the Raman spectrum evolution of a sample under study. We compare the power conversion efficiency and external quantum efficiency of P3HT(poly(3-hexylthiophene))-based PSCs to show the effect of the annealing on their performance. The dynamics of annealing the polymer-fullerene blends both at constant and slowly increasing temperature has been measured. Interestingly, the different fullerene derivatives results in very different annealing behaviour. We have also studied the dependence of the blend morphology on solvent and metallic contact coating. We also found out that the polymer crystallinity can depend significantly on the coordinate normal to the film surface. We discuss the applicability of the proposed technique for optimization of various donor-acceptor blends based on new polymer and low-molecular-weight organic materials for solar cells. This work was supported by M.V. Lomonosov Moscow State University Program of Development and by Russian Foundation for Basic Research grant #12-02-31599.